Abstract

In this paper a flexible hollow core waveguide for the terahertz spectral range is demonstrated. Its cladding is composed of a circular arrangement of dielectric tubes surrounded by a heat-shrink jacket that allows the fiber to be flexible. Characterization of straight samples shows that the hollow core allows the absorption caused by the polymethylmethacrylate tubes of the cladding to be reduced by 31 times at 0.375 THz and 272 times at 0.828 THz with respect to the bulk material, achieving losses of 0.3 and 0.16 dB/cm respectively. Bending loss is also experimentally measured and compared to numerical results. For large bending radii bending loss scales as
Rb−2, whereas for small bending radii additional resonances between core and cladding appear. The transmission window bandwidth is also shown to shrink as the bending radius is reduced. An analytical model is proposed to predict and quantify both of these bending effects.

Figures (5)

(a) Microscope image of the transverse cross section of the manufactured fiber along with its physical dimensions and dielectric properties. (b) Side picture of the fiber. (c) THz-TDS setup that have been used to characterize the straight fiber. (e) Frequency dependence of ℜ(nH) and ℑ(nH) measured experimentally for a PMMA disk.

(a) Top and middle panels show the numerical dispersion curves for two dielectric modes with different periodicity along the transverse direction, the core mode and hole modes. For the hole modes the color intensity is proportional to the power inside the hole regions normalized with respect to the total power of the mode. Experimental data is also shown. Bottom panel shows numerical propagation loss for the core mode with different values of ℑ(nH) and also experimentally measured loss. (c) Example images for the three different classes of modes supported by the waveguide.

(a) THz-TDS setup that have been used to characterize the 20 cm fiber sample for the bending. (b) A picture of the setup. (c) Experimental and numerical normalized transmission of the 20 cm TLF sample for different bending radii. (d) Comparison of propagation loss in case of bending for CTLFs with transparent and PMMA tubes.

(a) Effect of the conformal mapping on the refractive index of the fiber in the transverse direction. (b) Propagation loss of the core mode in the first transmission window for Rb = {4, 6, 8, 10} cm (from light to dark blue). (c) Propagation loss of the core mode in the second transmission window for Rb = {18, 22, 26, 30} cm (from light to dark violet). The resonance between core and hole mode is highlighted in the inset. (d) Evaluation of Eq. (4) for the first resonance edge (red) and of Eq. (5) for the resonances between core and hole modes (green) in the two transmission windows.